A tire air valve having the function of indicating tire pressure includes an air valve casing having an inner center post orderly installed thereon with a lower locking block, a first spring, an upper locking block and a second spring. A fixed member is secured on the upper locking block, and a cap is covered on the air valve casing and provided with an upper indicating member, a third spring, a lower indicating member and a locking ring. The upper indicating member has an outer wall formed into a low pressure region and a warning region, while the lower indicating member has an outer wall formed into a safety region. The upper and the lower locking block can completely seal up the air valve to prevent air from leaking, and a driver can judge whether tire pressure is normal by visual observation through the cap on the air valve casing.